Defect Inspection Method And System

ABSTRACT

An inspection system includes: a facility that uses wide-band illumination light having different wavelengths and single-wavelength light to perform dark-field illumination on an object of inspection, which has the surface thereof coated with a transparent film, in a plurality of illuminating directions at a plurality of illuminating angles; a facility that detects light reflected or scattered from repetitive patterns and light reflected or scattered from non-repetitive patterns with the wavelengths thereof separated from each other; a facility that efficiently detects light reflected or scattered from a foreign matter or defect in the repetitive patterns or non-repetitive patterns or a foreign matter or defect on the surface of the transparent film; and a facility that removes light, which is diffracted by the repetitive patterns, from a diffracted light image of actual patterns or design data representing patterns. Consequently, a more microscopic defect can be detected stably.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.11/626,925, filed Jan. 25, 2007, the contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and system for inspecting theoccurrence of a foreign matter or defect in a device manufacturingprocess, wherein a foreign matter existent on a thin-film substrate, asemiconductor substrate, or a photomask, or a defect occurring incircuit patterns is detected in the course of manufacturing asemiconductor chip or a liquid crystal product, and the detected foreignmatter or defect is analyzed in order to take measures.

2. Description of the Related Art

In a semiconductor manufacturing process, the presence of a foreignmatter on a semiconductor substrate (wafer) causes a defective such asimperfect insulation of wiring or a short circuit thereof. Furthermore,due to the trend of semiconductor devices to a more and more microscopicstructure, a more microscopic foreign mater brings about imperfectinsulation of a capacitor or destruction of a gate oxide film. Theforeign matters are mixed in various states because of various causes.Namely, the foreign matters may be produced by a movable unit includedin transportation equipment or a human body, produced in processingequipment due to reaction of a process gas or the like, or originallymixed in a chemical agent or material.

Likewise, in a process of manufacturing a liquid crystal display device,if a defective pattern is produced in a work due to a foreign matter,the work cannot be adopted as a display device. The same applies to amanufacturing process of a printed circuit board. The mixture of aforeign matter causes a short circuit or a defective connection inpatterns. Under such a background, in the case of semiconductormanufacturing, a plurality of foreign matter inspection systems may bedisposed relative to each production line. Thus, the presence of aforeign matter is discovered in the earliest possible stage and fed backto the manufacturing process, whereby a yield in manufacturing of asemiconductor device is improved.

As one of techniques for detecting a foreign matter on a semiconductorsubstrate, Japanese Unexamined Patent Publication No. 62-89336 (PriorArt 1) has disclosed a method capable of highly sensitively and highlyreliably inspecting the presence of a foreign matter and the occurrenceof a defect by discarding false or misleading information acquired frompatterns. According to the method, light scattered from a foreignmatter, which adheres to a semiconductor substrate, after laser light isirradiated to the semiconductor substrate is detected, and then comparedwith a result of inspection conducted on a semiconductor substrate ofthe same type that has been inspected immediately previously. Moreover,as disclosed in Japanese Unexamined Patent Publication No. 63-135848(Prior Art 2), a method is known in which light scattered from a foreignmatter, which adheres to a semiconductor substrate, after laser light isirradiated to the semiconductor substrate is detected, and the detectedforeign matter is analyzed using an analyzing technique such as laserphotoluminescence or secondary X-ray analysis (XMR).

Moreover, disclosed as a technique for inspecting the presence of aforeign matter is a method for irradiating coherent light to a wafer,removing light, which is emitted from repetitive patterns on the wafer,using a spatial filter, and intensifying light reflected from a foreignmatter or defect, which lacks repetitiveness, so as to detect theforeign matter or defect. Moreover, a foreign matter inspection systemthat irradiates light to a major group of straight lines, which isincluded in circuit patterns formed on a wafer, at an angle of 45°, forfear light having undergone zero-order diffraction due to the majorgroup of straight lines may enter an aperture for an objective lens isdescribed in Japanese Unexamined Patent Publication No. 1-117024 (PriorArt 3). In the Prior Art 3, a spatial filter is used to intercept lightreflected from a group of straight lines other than the major group ofstraight lines.

As prior arts concerning a defect inspection system and method forinspecting a foreign matter or the like, methods described in JapaneseUnexamined Patent Publications Nos. 1-250847 (Prior Art 4) and2000-105203 (Prior Art 5) are known. In particular, as for the Prior Art5, the patent publication describes that detective optical systems areswitched in order to change detectable pixel sizes. As for technologiesfor measuring the size of a foreign matter, a method is disclosed inJapanese Unexamined Patent Publication No. 2001-60607 (Prior Art 6).

SUMMARY OF THE INVENTION

However, the foregoing Prior Arts 1 to 5 cannot readily, highlysensitively, and quickly detect a microscopic foreign matter or defecton a substance on which repetitive patterns or non-repetitive patternscoexist. Namely, the Prior Arts 1 to 5 are disadvantageous in a pointthat detective sensitivity (a minimum dimension of a foreign matter tobe detected) is low except that for the repetitive patterns such asthose realizing memory cells or the like. Moreover, the Prior Arts 1 to5 are disadvantageous in a point that the detective sensitivity is lowrelative to a microscopic foreign matter or defect of about 0.1 μm in adiameter existing in a highly densely patterned area. Moreover, thePrior Arts 1 to 5 are disadvantageous in a point that the detectivesensitivity is low relative to a foreign matter or defect which causes ashort circuit of wiring, or a foreign matter shaped like a thin film.Moreover, the Prior Art 6 is disadvantageous in a point that theprecision in measurement of a foreign matter or defect is low. Moreover,the Prior Art 6 is disadvantageous in a point that the detectivesensitivity is low relative to a foreign matter on the surface of awafer coated with a transparent thin film.

The present invention addresses the foregoing problems. An object of thepresent invention is to provide a defect inspection method and systemcapable of inspecting an object of inspection on which repetitivepatterns and non-repetitive patterns coexist so as to detect amicroscopic foreign matter or defect quickly and highly precisely. Inparticular, the present invention provides a defect inspection methodand system capable of stably detecting a microscopic foreign matter ordefect while minimizing a difference in sensitivity between inspectiveareas, that is, a memory unit that includes many repetitive patterns anda logic unit in which non-repetitive patterns exist.

Moreover, another object of the present invention is to provide a defectinspection method and system that are attempted to improve thesensitivity in detecting a defect in a memory unit including manyrepetitive patterns. Herein, when a setting is designated forintercepting light, which is diffracted by patterns formed on asubstrate to be inspected, at a position of a Fourier transform in adetective optical system using a spatial filter, the setting may bedesignated by employing either light diffracted by actual patterns ordesign data representing patterns. The setting of the spatial filter isdesignated prior to inspection of each of a plurality of memory unitsincluded in the same die. When each of the memory units is inspected,the setting of the spatial filter is automatically designated in orderto highly sensitively detect a microscopic foreign matter or defect inthe memory unit.

To be more specific, in one aspect of the present invention, there isprovided a defect inspection system including: an illuminative opticalsystem capable of irradiating illuminating luminous fluxes, which areemitted from an illuminative light source and have differentwavelengths, to the surface of a substrate to be inspected in mutuallydifferent directions and at mutually different tilt angles; an objectivelens on which light reflected or scattered from a defect on thesubstrate to be inspected is converged; a detective optical systemrealized with an image-formation optical system that separates thewavelengths of the reflected or scattered light converged on theobjective lens from one another, and causes the different wavelengthcomponents to form images on the light receiving surfaces of respectivephotodetectors; and a signal processing system that converts imagesignals produced by the respective photodetectors included in thedetective optical system into digital image signals, and detects thedefect on the basis of the digital image signals produced by theconverter.

In another aspect of the present invention, the detective optical systemincluded in the defect inspection system includes a mechanical unit thatvaries an image-formation power by keeping constant the relativedistances between a Fourier-transform image, which is formed between theobjective lens and the image formation optical system, and the substrateto be inspected and photodetectors.

In another aspect of the present invention, the detective optical systemincluded in the defect inspection system includes a spatial filter thatis interposed between the objective lens and image formation opticalsystem and that has the ability to intercept light of a specificwavelength out of the light reflected or scattered from the substrate tobe inspected. Moreover, according to the present invention, the defectinspection system includes an arrangement that designates the conditionsfor the spatial filter, which is included in the detective opticalsystem and interposed between the objective lens and image formationoptical system, on the basis of design data representing patterns.

In another aspect of the present invention, there is provided a defectinspection system including: an illuminative optical system capable ofirradiating an illuminating luminous flux, which is emitted from anillumination light source, to the surface of a substrate to be inspectedat a high tilt angle and a low tilt angle that can be switched; adetective optical system including an objective lens which is disposedin a direction optimal for detecting a foreign matter or defect on theobject of inspection and on which light reflected or scattered from theforeign mater or defect is converged, an image formation optical systemthat causes the reflected or scattered light, which is converted on theobjective lens, to form an image, and a photodetector that converts thereflected or scattered light, which is caused to form an image by theimage formation optical system, into a signal; an A/D converter thatwhen the illuminative optical system illuminates the substrate to beinspected at the high and low tilt angles, converts an image signalproduced by the photodetector included in the detective optical systeminto a digital image signal; a defect detection unit that detects thedefect on the basis of the digital image signal produced by the A/Dconverter; and a means for verifying the detected foreign matter.

In another aspect of the present invention, there is employed a pulsedlaser, which emits light that falls within the ultraviolet region, as alow-angle illumination light source. Moreover, one pulse of laser lightemitted from the pulsed laser light source is split into a plurality ofpulses in order to decrease the peak value of the laser light. Theresultant laser light is irradiated to a specimen, whereby the damagethe specimen incurs is reduced.

The aforesaid and other objects of the present invention and thefeatures and advantages thereof will be apparent from the following moreparticular description of preferred embodiments of the inventionillustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram schematically showing the configuration of adefect inspection system in accordance with the first embodiment;

FIG. 2A is a side view for use in explaining the disposition of anilluminative optical system shown in FIG. 1;

FIG. 2B is a perspective view schematically showing the elements of alow-angle illumination optical system included in the first embodiment;

FIG. 3A is a perspective view showing a conically curved lens employedin an illuminative optical system that obliquely illuminates a lineararea 201-1 on a wafer;

FIG. 3B is a perspective view showing a cylindrical lens employed in anilluminative optical system that longitudinally illuminates the lineararea 201-1 on the wafer;

FIG. 3C is a perspective view showing a cylindrical lens employed in anilluminative optical system that laterally illuminates the linear area201-1 on the wafer;

FIG. 4A is a plan view of a high-angle illumination optical system;

FIG. 4B is a front view of the high-angle illumination optical system;

FIG. 4C is an A-A sectional view of the high-angle illumination opticalsystem;

FIG. 5A is a schematic front view of a variant of the high-angleillumination optical system;

FIG. 5B is a schematic front view of another variant of the high-angleillumination optical system;

FIG. 6 A is a schematic front view of still another variant of thehigh-angle illumination optical system;

FIG. 6B is a schematic front view of the variant shown in FIG. 6B;

FIG. 7 is a front view schematically showing the elements of anobjective lens;

FIG. 8A shows a diffraction pattern produced by light reflected orscattered from a wafer when a pupil of an objective lens is viewedthrough a pupil viewing optical system;

FIG. 8B shows a state in which the diffraction pattern observed throughthe pupil viewing optical system is intercepted by a one-dimensionalspatial filter;

FIG. 8C shows a diffraction pattern observed when the pupil of theobjective lens is viewed through the pupil viewing optical system andthe wafer is illuminated in multiple directions;

FIG. 8D shows a state in which a two-dimensional spatial filter isdisposed in order to intercept the diffraction pattern shown in FIG. 8C;

FIG. 9A is a plan view schematically showing the structure of a variableone-dimensional spatial filter;

FIG. 9B is a front view schematically showing the structure of thevariable one-dimensional spatial filter;

FIG. 9C is a front view schematically showing the structure of avariable two-dimensional spatial filter;

FIG. 9D is a plan view showing a spring structure included in thevariable two-dimensional spatial filter;

FIG. 10 is a plan view schematically showing the configuration of a chipof a semiconductor device formed on a wafer;

FIG. 11A is a front view for use in explaining in detail a detectiveoptical system;

FIG. 11B shows a waveform of a detective signal produced when light S0reflected or scattered from a wafer is detected;

FIG. 11C shows a waveform of a detective signal produced when lightreflected or scattered under illumination with laser light is selectedand detected;

FIG. 12 is a flowchart describing an inspection procedure;

FIG. 13 is a block diagram showing in detail a signal processing systemincluded in the first embodiment;

FIG. 14 is a block diagram showing in detail the configuration of athreshold calculation block included in the first embodiment;

FIG. 15 is a perspective view showing the overall configuration of adefect inspection system in accordance with the second embodiment;

FIG. 16 schematically shows a defect inspection system in accordancewith the third embodiment including an observational optical system;

FIG. 17 is a plan view showing the arrangement of elements of a pathbranching optical system included in the fourth embodiment;

FIG. 18A is a block diagram schematically showing the path branchingoptical system included in the fourth embodiment;

FIG. 18B shows a waveform of a signal of pulsed laser light emitted froma laser light source;

FIG. 18C shows a waveform of pulses demonstrating that one pulse oflaser light emitted from the laser light source is split into twopulses;

FIG. 19A is a block diagram schematically showing the elements of a pathbranching optical system included in a variant of the fourth embodiment;and

FIG. 19B shows a waveform of pulses demonstrating split of a pulse.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, embodiments of the present invention will bedescribed below.

A defect inspection system in accordance with the present inventionhighly sensitively and quickly inspects the presence of various defectsincluding a foreign matter, a defective pattern, and a micro-scratch ona substrate to be inspected such as a wafer that is made of variousmaterials through various manufacturing steps, and especially stablydetects a defect on the surface of a thin film coated over the surfaceof the wafer independently of a defect in the thin film.

Specifically, the defect inspection system in accordance with thepresent invention is designed so that an angle α of irradiation and adirection φ of irradiation at or in which a slit-shaped beam 201 thatis, as shown in FIG. 2, produced for illumination by an illuminativeoptical system 10 can be varied depending on an object of inspection. Asshown in FIG. 1, the elements of a detective optical system 20 arearranged so that the surface of the object of inspection and the lightreceiving surface of a detector 26 will have a relationship of imageformation. Moreover, a power for image formation offered by thedetective optical system 20 is made variable so that the size of adefect-detected pixel can be determined according to the size of adefect to be detected. Thus, inspection is carried out.

Furthermore, the defect inspection system in accordance with the presentinvention includes a facility that distinguishes a kind of detecteddefect by recognizing a difference in light, which is scattered from adefect by irradiating illumination light waves at different angles ofirradiation, as a characteristic quantity.

The embodiments of the defect inspection system in accordance with thepresent invention will be described concretely. The embodiments will bedescribed on the assumption that a defect such as a small or largeforeign matter or a micro-scratch on a semiconductor wafer or atransparent film coated over the wafer, a foreign matter in thetransparent film, or a defective pattern is inspected. However, thepresent invention is not limited to the semiconductor wafer but can beapplied to a thin-film substrate, a photomask, a thin-film transistor(TFT), or a plasma display panel (PDP).

First Embodiment

FIG. 1 shows the configuration of a defect inspection system inaccordance with the first embodiment. The defect inspection systemincludes mainly an illuminative optical system 10, a variable-powerdetective optical system 20, a transportation system 30, a signalprocessing system 40, and an overall control unit 50 that controls theentire defect inspection system.

The transportation system 30 includes an X stage 31-1, a Y stage 21-2, aZ stage 32, and a θ stage 33 that are used to move a placement tablebearing a substrate 1 to be inspected such as a wafer that is made ofvarious materials through various manufacturing steps, and a drivecircuit 35 that controls the stages.

The illuminative optical system 10 includes an illumination light source12 whose light falls in a wide wavelength band, a laser light source 11,a beam enlargement optical system 16, mirrors 254 and 256, and a lens255. Light emitted from the laser light source 11 is enlarged to acertain size by the beam enlargement optical system 16, and thenirradiated to the substrate 1 to be inspected in a plurality of obliquedirections via the mirror 254, lens 255, and mirror 256.

The detective optical system 20 includes an objective lens 21, a spatialfilter 22, an image formation lens 23, an optical filter 25, a beamsplitter 29, and photodetectors 26 a and 26 b such as time-delayintegration (TDI) image sensors.

Processing circuits 40 a and 40 b included in the signal processingsystem 40 manipulate image signals produced by the photodetectors 26 aand 26 b respectively so as to detect a defect or foreign matter. Thebeam splitter 29 reflects light that falls in a specific wavelength bandand transmits light having the other wavelengths.

An observational optical system 60 includes a lens 61, a polarizationbeam splitter 62, an illumination light source 63, and an imaging means64. Herein, p-polarized light emitted from the illumination light source63 is reflected from the polarization beam splitter 62, has thus thepath thereof angled toward the wafer 1, and is then converged on thelens 61. Consequently, the surface of the wafer 1 is illuminated. Out oflight reflected or scattered from the wafer 1, light incident on thelens 61 falls on the polarization beam splitter 62. The s-polarizedlight component of the light passes through the polarization beamsplitter 62, and forms an image which is picked up by the imaging means64. The wafer 1 is inspected in advance by other inspection system, andthe presence or absence of a detected foreign matter and the shapethereof are verified through the observational optical system. Moreover,the observational optical system is used to observe a foreign matter ordefect detected by the variable-power detective optical system 20.

The overall control unit 50 designates conditions for inspection or thelike, and controls the illuminative optical system 10, variable-powerdetective optical system 20, transportation system 30, and signalprocessing system 40. The overall control unit 50 includes aninput/output means 51 (including a keyboard and a network), a displaymeans 52, and a memory unit 53. Reference numeral 55 denotes a storagemeans (server) in which design data including data that representscircuit patterns formed on the surface of the substrate 1 to beinspected is stored. The design data is used to form a spatial opticalimage.

The defect inspection system includes an automatic focusing controlsystem (not shown) so that an image of the surface of the wafer 1 willbe formed on the light receiving surfaces of the respectivephotodetectors 26 a and 26 b.

The present inspection system is designed to be able to illuminate thesurface of the substrate 1 to be inspected in a plurality of directions.As shown in FIG. 2A, the illuminative optical system 10 includes, asdescribed in the Japanese Unexamined Patent Publication No. 2000-105203,the beam enlargement optical system 16 composed of, for example, aconcave lens and a convex lens that are not shown, a lens 14 thatreshapes light L0 emitted from the laser light source 11 into aslit-shaped beam, and a mirror 15. The illuminative optical system 10reshapes the light L0, which is emitted from the laser light source 11,into the slit-shaped beam 201, and irradiates the slit-shaped beam to aslit-shaped area 201-1 on the wafer 1.

The inspection system of the present embodiment includes, as anarrangement for illuminating the surface of the wafer 1 with asingle-wavelength laser beam at a low angle (low angle of incidence), anarrangement that irradiates the slit-shaped beam 201 (light irradiatedto the slit-shaped area 201-1 on the wafer and referred to as theslit-shaped beam) to the wafer 1 (substrate to be inspected) on thespecimen placement table 34 on a planar basis in a plurality ofdirections (four directions 220, 230, 240, and 250 in FIG. 2B) and at aplurality of illuminating angles (angles α, β, and γ in FIG. 2B).

The reason why illumination light is reshaped into the slit-shaped beam201 is that an image carried by light that is scattered from a foreignmatter or defect under illumination is formed on the detective surfacesof the photodetectors 26 defined by the respective arrays of lightreceiving elements, and thus detected comprehensively in order to speedup inspection of a foreign matter.

Specifically, the θ stage 33 is driven so that the directions, in whichthe chips 22 constituting the wafer 1 are arrayed, will be parallel tothe scanning direction of the X stage 31-1 and the scanning direction ofthe Y stage 31-2. Thus, the orientation of the wafer 1 placed on theplacement table 34 is adjusted, and the slit-shaped beam 201 isirradiated to the wafer 1 having the orientation thereof adjusted.

The slit-shaped area 201-1 on the wafer 1 to which the slit-shaped beam201 is irradiated is defined by adjusting the ray axis of the beam sothat it will be perpendicular to the scanning direction X of the X stage31-1 (the longitudinal direction of the slit-shaped beam 201 irradiatedto the wafer 1 will be perpendicular to the scanning direction X of theX stage 31-1), parallel to the scanning direction Y of the Y stage 31-2(the longitudinal direction of the slit-shaped beam 201 irradiated tothe wafer 1 will be parallel to the scanning direction Y of the Y stage31-2), and parallel to the direction in which the pixel locations in thephotodetectors 26 a and 26 b are arrayed. This is advantageous in thatwhen image signals representing the surfaces of chips are compared witheach other, the chips can be readily aligned with each other. Theslit-shaped beam 201 can be produced by inserting a conically curvedlens 14, 224, or 234 shown in FIG. 3A, a cylindrical lens 255 shown inFIG. 3B, or a cylindrical lens 244 shown in FIG. 3C into a path.

For illuminations achieved in the directions 220 and 230 respectivelyshown in FIG. 2B, laser light shaped like a slit is irradiated to theslit-shaped area 201-1 on the wafer 1 in a direction that is laterallydeviated by an angle φ from the Y-axis direction of the wafer and thatmeets the surface of the wafer 1 at an angle α (in FIG. 2B, a path alongwhich illumination light propagating in the direction 230 is reflectedfrom the mirror 233, passes through the cylindrical lens 234, andreaches the mirror 236 is shown superimposed on a path extending fromthe mirror 236 to the area 201-1 on the wafer 1 to which the slit-shapedbeam 201 is irradiated).

As an arrangement for realizing the foregoing illuminations, theconically curved lens 14 whose radius of curvature relative to thelongitudinal direction varies continuously (corresponding to the lens224 or 234 shown in FIG. 2B) is, as shown in FIG. 3A, disposed in thepath so that the major-axis direction of the slit-shaped beam 201 thatis irradiated to the wafer 1 will be parallel to the scanning directionof the Y stage 31-2 (the mirrors 225, 226, 235, and 236 are excludedfrom FIG. 3).

As for the illuminations achieved in the directions 240 and 250, sincethe illuminations are achieved in the same directions as the scanningdirections of the X stage 31-1 and Y stage 31-2 respectively or thedirections perpendicular to the scanning directions, the lens 255 shownin FIG. 3B (mirror 245 is excluded) or the lens 244 shown in FIG. 3C isused to produce the slit-shaped beam 201 so as to illuminate theslit-shaped area 201-1 on the wafer (in FIG. 3B and FIG. 3C, the lenses244 and 255 are shown as cylindrical lenses but are not necessarilycylindrical when a shift of a position on which light irradiated to thewafer 1 is focused has to be compensated).

As shown in FIG. 2A, the mirror 15 (corresponding to the mirror 226 or236 shown in FIG. 2B) and the mirror 205 (corresponding to the mirror225 or 235 shown in FIG. 2B) are mechanically switched in response to acommand issued from the overall control unit 50. Thus, an illuminatingangle α can be varied depending on a kind of foreign matter to bedetected on the substrate 1 to be inspected. As shown in FIG. 2C,whatever illuminating angle is designated, the slit-shaped area 201-1 onthe wafer 1 to which the slit-shaped beam 201 is irradiated covers thedirection 203 in which the pixel locations included in thephotodetectors 26 a and 26 b are arrayed. In whatever directionsincluding directions opposite to the directions 220 and 230 illuminationis achieved, the position illuminated by the slit-shaped beam 201corresponds to the slit-shaped area 201-1 on the wafer 1.

Consequently, illumination can be achieved at an angle φ of about 45°with light including rays parallel to the Y direction. In particular,when the slit-shaped beam 201 is composed of rays parallel to the Ydirection, diffracted light emanating from circuit patterns whose majorgroup of straight lines is oriented in the X and Y directions isintercepted by the spatial filter 22.

Incidentally, methods of manufacturing the conically curved lens 14include a method described in, for example, Japanese Unexamined PatentPublication No. 2000-105203.

The reason why the slit-shaped beam 201 is irradiated to the slit-shapedarea 201-1 on the wafer 1 at a plurality of illuminating angles is todetect various kinds of foreign matters present on the surface of thewafer 1. Specifically, a defective pattern on the substrate 1 to beinspected or a short foreign matter is detected.

A more microscopic foreign matter or defect must be detected in aportion of each of chips 202 constituting the wafer 1 in which the samepattern is repeatedly formed at intervals of a relatively narrow pitchin order to realize a memory unit or the like. A method conventionallyknown for detection of the microscopic foreign matter or defect is suchthat: relatively intense light is fed to the surface of the wafer 1 at alow angle with respect to the surface of the wafer; and diffracted lightemanating from repetitive patterns formed closely at intervals of arelatively narrow pitch is intercepted by the spatial filter 22 in orderto detect light reflected or scattered from the foreign matter or defecton the surface of the wafer 1. As a light source of the relativelyintense light to be used to illuminate the surface of the wafer 1 at thelow angle, a laser is suitable.

However, for detection of a foreign matter or defect in non-repetitivepatterns formed to realize a logic unit or the like or looselyrepetitive patterns formed at intervals of a relatively coarse pitch,since light scattered from the non-repetitive patterns or the looselyrepetitive patterns formed at intervals of a relatively coarse pitchcannot be fully intercepted by the spatial filter, it is hard to extractonly the light scattered from the foreign matter or defect from lightreflected or scattered from the wafer 1. For detection of the foreignmatter or defect in the non-repetitive patterns formed to realize alogic unit or the like, the wafer 1 should be illuminated in a directionof a relatively high angle. Thus, images of adjoining chips are detectedand then compared with each other. However, when an attempt is made todetect a foreign matter or defect on the wafer 1 coated with anoptically transparent film using a laser light source, since laser lightfalls within an extremely narrow range of wavelengths, if the thicknessof the transparent film coated over the surface of the wafer varies toexhibit a distribution, the intensity of light reflected from thesurface changes due to interference. This disables stable detection of adefect. According to the present embodiment, for detection of a foreignmatter or defect in repetitive patterns, a light source whose lightfalls within a wide wavelength band is used to illuminate the wafer 1 ina direction of a relatively high angle in efforts to avoid the influenceof the variation in the thickness of the transparent film coated overthe surface of the wafer 1.

The present embodiment has an arrangement for realizing high-angleillumination that is achieved with light falling within a widewavelength band and low-angle illumination that is achieved withsingle-wavelength light.

To begin with, a low-angle illumination optical system will be describedin conjunction with FIG. 2A to FIG. 3C. FIG. 2A is an explanatorydiagram concerning the principles of an arrangement that is thelow-angle illumination optical system. With an increase in anilluminating angle α for low-angle illumination, an amount of lightreflected and diffracted by circuit patterns increases, and asignal-to-noise ratio decreases. Therefore, an experimentally obtainedoptimal value is adopted. For example, for detection of a low foreignmatter on the surface of a wafer, the illuminating angle α should besmall, for example, should range from 1° to 10°, or preferably, from 1°to 5°.

As for an illuminating direction φ for low-angle illumination, forexample, at a wiring step, the direction wiring patterns formed on awafer and the illuminating direction are aligned with each other. Thismakes it easier to detect a foreign matter in wiring. Moreover, whencircuit patterns on a wafer are not wiring patterns but consist ofcontact holes and capacitors, the circuit patterns have no particulardirectivity. Therefore, a chip containing the circuit patterns should beilluminated in a direction of about 45°. Talking of changing ofilluminating angles, in the case of low-angle illumination, the twomirrors 15 and 205 whose angles are different from each other are, forexample, as shown in FIG. 2A, switched. Otherwise, the angle of themirror 15 (or 205) may be changed with the X direction (perpendicular tothe sheet of paper of the drawing) as an axis of rotation using arotating means that is not shown. At this time, the mirror 15 is movedeven in the Z direction so that the ray axis of the slit-shaped beam 201will be aligned on a wafer with the ray axis of light detected by thedetective optical system. Moreover, the lens 14 is also moved in the Zdirection so that the slit-shaped beam 201 will have a minimum diameteron the ray axis of the light detected by the detective optical system.

Next, a method for changing illuminating directions will be described inconjunction with FIG. 2B.

A branching optical element 218 shown in FIG. 2B is composed of amirror, a prism, and others. A driving means that is not shown is usedto move the branching optical element 218 in the Y direction, wherebylaser light L0 emitted from the laser light source 11 is transmitted orreflected to propagate in any of three directions. Laser light L1transmitted by the branching optical element 218 is branched intotransmitted light and reflected light by a half prism 221. For example,after the transmitted light passes through a wave plate 236, it isreflected by a mirror 236 via a mirror 231, a beam diameter correctionoptical system 232, a mirror 233, and a conically curved lens 234.Consequently, the slit-shaped beam 201 enters the wafer 1 in thedirection 230. Incidentally, the mirror 235 is disposed so that it canbe inserted into or withdrawn from the space between the conicallycurved lens 234 and mirror 236, whereby an angle α of incidence at whichthe slit-shaped beam 201 formed by the conically curved lens 234 fallson the slit-shaped area 201-1 on the wafer can be changed from one toanother.

On the other hand, light reflected from the half prism 221 is handled byan optical element having the same ability as the foregoing ability sothat the slit-shaped beam 201 will be irradiated to the wafer 1 in thedirection 220. Even in this path, a mirror 225 is disposed so that itcan be inserted into or withdrawn from the space between a conicallycurved lens 224 and a mirror 226. Thus, an angle α of incidence at whichthe slit-shaped beam 201 formed by the conically curved lens 224 fallson the slit-shaped area 201-1 on the wafer can be changed from one toanother. Incidentally, beam diameter correction optical systems 222 and232 adjust the diameters of laser light waves incident on the conicallycurved lenses 234 and 224 respectively so that the slit-shaped beams 201to be irradiated to the wafer 1 will have the same size. Moreover, if amirror 260 is substituted for the half prism 221, illumination isachieved in the direction 220 alone. If neither the half prism 221 northe mirror 260 is used, illumination is achieved in the direction 230alone. Moreover, a wave plate 226 or 236 located behind the half prism221 may be used to designate a direction of polarization of laser lightto be irradiated so as to produce, for example, p-polarized light ors-polarized light alone.

After laser light L2 reflected from the branching optical element 218passes through the beam diameter correction optical system 241, it isreflected from the mirrors 242 and 243, transmitted by a cylindricallens 244 (see FIG. 3C), and reflected by a mirror 245. Consequently, abeam reshaped like a slit by the cylindrical lens 244 is irradiated tothe slit-shaped area 201-1 on the wafer 1 in the direction 240.

On the other hand, laser light L3 is reflected by a mirror 251 in thesame manner as light L2 is reflected by the foregoing optical elementdisposed on the path, and transmitted by a beam diameter correctionoptical system 252. Thereafter, the laser light is reflected frommirrors 253 and 254, transmitted by a cylindrical lens 255, andreflected by a mirror 256. Consequently, the slit-shaped beam 201 isirradiated to the wafer 1 in the direction 250.

As for the illuminating directions 240 and 250, if many wiring patternsformed on a wafer are parallel to the X or Y direction, the directionsof illumination can be aligned with the X or Y direction at, forexample, a wiring step. This is advantageous in that a foreign matter inwiring can be readily detected.

As the laser light source 11, a high-power YAG laser whose laser lightcontains a second harmonic having a wavelength of 532 nm is adopted.However, the wavelength need not always be 532 nm, and the fourthharmonic of the laser light having a wavelength of 266 nm may beutilized. Moreover, an ultraviolet, far-ultraviolet, orvacuum-ultraviolet laser, an Argon laser, a nitrogen laser, a He—Cdlaser, an eximer laser, or a semiconductor layer may be adopted as alight source.

In general, when the wavelength of laser light is shortened, theresolution of a detected image improves. This permits high-sensitivityinspection.

On the other hand, FIG. 4 shows in detail illumination to be achievedusing a wide wavelength band, though the illumination light source 12 tobe used for the illumination is illustrated in FIG. 1. In FIG. 4A toFIG. 4C, light falling within a wide wavelength band and emanating fromthe light source 12 is, as shown in FIG. 4A, branched into three pathsby half-silvered mirrors 1801 and 1802 included in a high-angleillumination optical system 18. After the light waves have their pathsangled by mirrors 1803 to 1811, the light waves are, as shown in FIG.4B, converged on a group of condenser lenses 1813. Consequently, thesubstrate 1 to be inspected is illuminated in eight directions, whichare equiangular directions with θ=45° between adjoining ones, by a groupof mirrors 1824 to 1831. The transmittances of the mirrors 1801 to 1805,1807, 1809, and 1810 disposed along the paths are determined so that theintensities of illuminations in the respective directions will be equalto one another on the substrate 1 to be inspected.

FIG. 5A and FIG. 5B show other examples of high-angle illumination. FIG.5A shows an arrangement in which: a wavelength selection filter 2002selects light, which falls within a specific range of wavelengths, fromlight that is emitted from a wide-band lamp light source 12 and that istransmitted by a lens 2001; after transmitted by a lens 2003, theselected light is routed to a half-silvered mirror 2005, which isinterposed between the objective lens 21 and the spatial filter 22 orbetween the spatial filer 22 and the image formation lens 23, viacollimator lenses 2004, and then irradiated using spherical mirrors 2006so that the numerical aperture of the objective lens 21 will not beimpaired. FIG. 5B shows an arrangement for irradiating light, which isemitted from the wide-band lamp light source 12, as it is withoutselecting a specific wavelength. For detection of a foreign matter inwiring at a wiring step or detection of a defective pattern, anilluminating angle η should be increased. However, in consideration ofthe relationship between a pattern and a signal-to-noise ratio for asignal acquired from a foreign matter, the illuminating angle η shouldrange from 40° to 60°, or preferably, from 45° to 55°. Moreover, if astep of handling an object of inspection is associated with a kind offoreign matter to be detected, whatever illuminating angle is selectedmay be designated in an inspection plan.

Moreover, for detection of all foreign matters or defective patterns onthe surface of a wafer without leaving any foreign matter or defectivepattern uninspected, the illuminating angle for high-angle illuminationdescribed in conjunction with FIG. 4 or FIG. 5 may be set to anintermediate range of the aforesaid range of values, that is, a rangefrom 5° to 45°.

FIG. 6A and FIG. 6B show still other examples of high-angleillumination. As shown in FIG. 6A, light emitted from the light source12 is propagated through a wavelength selection filter 2101, split intoeight light waves by optical fibers 2102 to 2109, and converged onlenses 2112 to 2119 attached to the emission ends of the respectiveoptical fibers. The light waves are then, as shown in FIG. 6B, reflectedfrom a group of mirrors 2120 in order to change their paths.Consequently, high-angle illumination is performed on the wafer 1.

Next, the detective optical system 20 will be described below.

The detective optical system 20 is configured so that light reflectedand diffracted by the substrate 1 to be inspected such as a wafer isdetected by the photodetectors 26 a and 26 b, which are realized withTDI image sensors or the like, via the objective lens 21, spatial filter22, image formation lens (variable-power image formation optical system)23, optical filter 25 composed of a density filter and a sheetpolarizer, and beam splitter 29. When the TDI sensors are adopted as thephotodetectors 26 a and 26 b, the TDI sensors each having a plurality ofoutput taps provide a plurality of signals concurrently. The signalprocessing system 40 manipulates the plurality of signals concurrentlyusing a plurality of processing circuits or a plurality of pieces ofprocessing software. Consequently, a defect can be detected quickly.

The spatial filter 22 has the ability to intercept a Fourier-transformimage carried by light reflected and diffracted by repetitive patternson the wafer 1 and pass light scattered from a defect or foreign matter.The spatial filter 22 is disposed in an area where it helps theobjective lens 21 offer the spatial frequency, that is, at the positionof an image plane on which a Fourier transform is formed (correspondingto an exit pupil). The spatial filter 22 exhibits an opticalcharacteristic of intercepting a specific wavelength alone andtransmitting the other wavelengths. Specifically, the spatial filter 22intercepts light, which is diffracted by repetitive patterns formed in amemory unit or any other area and is derived from illumination lightirradiated to the substrate 1 to be inspected, and passes lightscattered from a defect or foreign matter. Moreover, the spatial filter22 passes light diffracted by non-repetitive patterns on the surface ofthe substrate 1 to be inspected that is illuminated by the wide-bandillumination light source 12. Light passing through the spatial filter22 is separated into different wavelengths by the beam splitter 29. Awavelength identical to the wavelength of light emitted from the laserlight source 11 is reflected to reach the photodetector 26 a, and theother wavelengths are transmitted to reach the photodetector 26 b. Thismakes it possible to avoid the saturation of each photodetector in termsof an amount of received light which is caused by light diffracted bythe non-repetitive patterns.

Incidentally, a pupil viewing optical system 70 that is composed of amirror 90 which can withdraw in the Y direction during inspection, aprojection lens 91, and a TV camera 92 and that is disposed along theoptical axis of the detective optical system 20 is used to image brightpoints 502 (drawn with filled circles in FIG. 8A) in an image, which iscarried by light reflected and diffracted by repetitive patterns and isviewed at the position of the image plane of a Fourier transform, withina field of view 501 offered by the pupil viewing optical system 70. Asheet interceptor 503 including a rectangular interceptor like the oneshown in FIG. 8B is disposed at the position of the image plane of aFourier transform. Both ends of the sheet interceptor 503 are, as shownin FIG. 9D, fixed to springs 715 and 716. The resultant sheetinterceptor 503 is fixed to an arm 710 that includes, as shown in FIG.9A and FIG. 9B, a motor 725, and a pair of members 710 a and 710 bcapable of being opened or closed in the X direction by means of a feedscrew 720 and a slide guide 722. A spacing Wx between adjoining ones ofslats included in the sheet interceptor 503 is mechanically varied, andthe entire sheet interceptor is moved along a guide 728 by means of amotor 726 and a feed screw 737 (in directions Ox in FIG. 8B). Thus, therectangular interceptor is adjusted so that the spacing betweenadjoining slats will be equal to a pitch Px between adjoining ones ofthe bright points 502 in the reflected and diffracted light image.Specifically, the sheet interceptor 503 is, as shown in FIG. 8B,adjusted for fear the bright points in the image of the light reflectedand diffracted by the repetitive patterns, which is viewed at theposition of the image plane of a Fourier transform, may come out of thesheet interceptor 503.

As mentioned above, the spacing Wx between adjoining ones of the slatsincluded in the sheet interceptor 503 is determined based on an actualimage of patterns observed through the pupil viewing optical system 70.Moreover, design data representing patterns may be used to create aFourier-transform image, and the spacing may be determined based on theimage. Moreover, a sheet interceptor 503′ whose interceptive width isdifferent from that of the sheet interceptor 503 may be attached to thearm 710. In this case, if a table 740 bearing the entire arm 710 ismoved in the Y direction along a guide 732 by means of a motor 735 and afeed screw 730 in order to switch the sheet interceptors, a change inthe size of bright points in a reflected and diffracted light image canbe coped with. The switching is performed in response to a command sentfrom the overall control unit 50 after a signal acquired by the TVcamera 92 is manipulated by the signal processing system 95.Incidentally, the sheet interceptor 503 may not be employed.Alternatively, based on an image signal produced by the TV camera 92, aninterceptor may be formed on a transparent substrate included in, forexample, a liquid crystal display device, with black and white reversed,and substituted for the sheet interceptor 503.

FIG. 8C shows an image (bright points 507) that is carried by lightreflected and diffracted by repetitive patterns and that is viewed atthe position of the image plane of a Fourier transform within a field ofview 501 offered by the pupil viewing optical system 70 when the wafer 1is illuminated simultaneously in different directions. When the brightpoints 507 are laterally widened, a two-dimensional interceptive patternthat is, as shown in FIG. 8D, a combination of a lengthwise interceptivepattern 503 and a sideways interceptive pattern 504 is used to interceptthe bright points 507 in the image carried by the light reflected anddiffracted by the repetitive patterns formed on the wafer 1.

A sheet interceptor 504 having, as shown in FIG. 9C, the same structureas the one described in conjunction with FIG. 9A and FIG. 9B is includedand superimposed on the sheet interceptor 503 so that the slatsextending in the X and Y directions will intersect one another. In thiscase, the sheet interceptor 503 is placed on the bottom of the arm 710including the pair of members that can be opened or closed, while thesheet interceptor 504 is placed on the top of the arm 710 so that itwill jut out. Consequently, the interceptors will not be defocused inthe Z direction on the plane containing the pupil of the objective lens21.

Owing to the above arrangement, by adjusting the spacing Wx in the Xdirection between adjoining ones of the slats included in the sheetinterceptor 503, the spacing Wy in the Y direction between adjoiningones of the slats included in the sheet interceptor 504, and the phasesOx and Oy of the sheet interceptor assembly, the spacings in therespective directions between adjoining ones of the slats included inthe two-dimensional interceptive pattern made by combining thelengthwise interceptive pattern 503 and the sideways interceptivepattern 504 can be, as shown in FIG. 8D, adjusted.

Consequently, as shown in FIG. 10, even when a plurality of memory unitsA to D exists in a chip, the spacings Wx and Wy between slats includedin the interceptor included in the spatial filter are preserved duringdesignation of conditions prior to inspection, and then designatedimmediately before the position of an area to be inspected is focusedon. Thus, a microscopic defect can be detected highly sensitively.

In the present embodiment, the detective optical system 20 is designedto detect light, which is reflected or scattered from the wafer 1 afterthe wafer 1 is illuminated by irradiating laser light, which is emittedfrom the laser light source 11, via the low-angle illumination system asdescribed in conjunction with FIG. 2A to FIG. 3C, separately from lightthat is reflected or scattered from the wafer 1 illuminated with light,which is emitted from the wide-band light source 12 and falls within awide wavelength band, via the high-angle illumination system asdescribed in conjunction with FIG. 4A to FIG. 6B. Specifically, the beamsplitter 29 shown in FIG. 1 reflects light that is reflected orscattered from the wafer 1 illuminated with light irradiated via thelow-angle illumination system. The photodetector 26 a detects thereflected light. The beam splitter 29 transmits light that is reflectedor scattered from the wafer 1 illuminated with light irradiated via thehigh-angle illumination system, and the photodetector 26 b detects thetransmitted light.

FIG. 11A shows the detective optical system in detail. Herein, referencenumeral S0 denotes light that is reflected or scattered from the wafer 1and transmitted by the objective lens 21 and image formation lens 23. Inthe light, light reflected or scattered from the wafer 1 afterirradiated via the low-angle illumination system, and light reflected orscattered from the wafer 1 after irradiated via the high-angleillumination system are mixed. The surface of the beam splitter 29 iscoated with a thin film 2901 having a property of reflecting light whosewavelength is identical to that of laser light emitted from the laserlight source 11. Among the components of the light SO reflected orscattered from the wafer 1, a reflected or scattered light component S1of laser light emitted from the laser light source 11 is reflected fromthe beam splitter 29 and then routed to the photodetector 26 a. Thedetecting surface of the photodetector 26 a (not shown) is located atthe position of the image plane of an image formed by the imageformation lens 23. The photodetector 26 a detects an image of lightreflected or scattered from the wafer 1 and derived from low-angleillumination achieved by the laser. On the other hand, the thin film2901 coated over the surface of the beam splitter 29 transmits lighthaving wavelengths other than the same wavelength as the wavelength oflaser light. Therefore, a light component S2 reflected or scattered fromthe wafer 1 and derived from high-angle illumination achieved with lightfalling within a wide wavelength band enters the photodetector 26 blocated at the position of the image plane of an image formed by theimage formation lens 23. The photodetector 26 b detects a reflected orscattered light image derived from high-angle illumination achieved withlight emitted from the wide-band light source 12.

Assuming that the light S0 reflected or scattered from the wafer 1 isnot separated into the components S1 and S2 but detected as it is by thephotodetector 26 a, the detective signal has a defect signal 950 buriedin noises as shown in FIG. 11B. On the other hand, when the component S1is separated from the light S0 reflected or scattered from the wafer 1and then detected by the photodetector 26 a as it is in the presentembodiment, the detective signal has a defect signal 960 distinguishedas shown in FIG. 11C. Thus, the defect signal can be detected.

On the other hand, light emitted from the wide-band light source 12 forhigh-angle illumination is set to a lower intensity than light emittedfrom the laser light source 11 for low-angle illumination is. Thephotodetector 26 b that detects light reflected or scattered from thewafer 1 and derived from low-angle illumination achieved by thewide-band light source 12 is set to a higher sensitivity than thephotodetector 26 a is. At this time, if the light component S1 were notseparated from the light S0 reflected or scattered from the wafer 1 butwere detected by the photodetector 26 b, the photodetector 26 b might besaturated due to the component S1. However, in the present embodiment,since the photodetector 26 b detects the light S2 that remains intactwith the component S1 separated from the light S0 reflected or scatteredfrom the wafer 1, the photodetector will not be saturated but can stablydetect light.

In the present embodiment, relatively high-luminance light reflected orscattered from a foreign matter on the surface of the wafer 1 andderived from low-angle illumination achieved with relativelyhigh-luminance laser light emitted from the laser light source 11 isdetected separately from light that is reflected or scattered from thesurface of the wafer 1 and that is derived from high-angle illuminationachieved with wide-band light emitted from the wide-band light source12. Therefore, low-angle illumination with laser light and high-angleillumination with wide-band light can be achieved simultaneously.Consequently, the whole of the wafer 1 can be inspected during oneinspection. Namely, both an area on the wafer 1 having repetitivepatterns formed therein at intervals of a relatively small pitch (forexample, patterns in a memory unit) and an area having patterns formedtherein at intervals of a relatively large pitch (for example,non-memory patterns in a logic unit) can be continuously inspected todetect a defect without the necessity of switching optical systems.

The inspection system of the present embodiment supports both a mode inwhich foreign matter inspection is performed quickly and a mode in whichhigh-sensitivity inspection is performed slowly. For an object ofinspection or an area in which circuit patterns are formed at a highdensity, the power of the detective optical system is raised in order toacquire a high-resolution image signal. This permits highly sensitiveinspection. Moreover, for an object of inspection or an area in whichcircuit patterns are formed at a low density, the power of the detectiveoptical system is lowered in order to perform inspection quickly withhigh sensitivity maintained.

Consequently, the size of a foreign matter that should be detected andthe size of a defect-detected pixel can be optimized. Noises caused byanything other than a foreign matter are removed, and only lightscattered from the foreign matter can be detected efficiently. Namely,in the inspection system in accordance with the present embodiment, thepower of the detective optical system 20 located above the wafer 1 canbe varied using a simple arrangement.

Next, an arrangement for varying the power of the detective opticalsystem and movements to be performed will be described in conjunctionwith FIG. 7.

The power of the detective optical system is varied in response to acommand issued from the overall control unit 50. The image formationlens 23 includes movable lenses 401, 402, and 403 and a moving mechanism404. When the power is varied, the magnification of a wafer surfaceimage formed on the photodetector 26 a or 26 b can be varied without thenecessity of changing the positions of the objective lens 21 and spatialfilter 22 in an optical-axis direction. In other words, the relativepositions between the substrate 1 to be inspected and the photodetector26 a or 26 b need not be changed even at the time of varying the power.The moving mechanism 404 having a simple structure can be used to varythe power. Furthermore, since the size of the image plane of a Fouriertransform remains unchanged, the spatial filter 22 need not be modified.

Assuming that f₁ denotes the focal length of the objective lens 21 andf₂ denotes the focal length of the image formation lens 23, the power Mof the detective optical system 20 is calculated according to formula 1presented below.

M=f ₂ /f ₁  (1)

In order to realize the power M of the variable-power detective opticalsystem 20, since f₁ denotes a fixed value, the movable lenses are movedto positions permitting f₂ to assume the product of M by f₁.

FIG. 7 shows the configuration of the image formation lens 23 includingthe movable lenses 401 to 403 and the moving mechanism 404, and shows anarrangement for moving the movable lenses 401, 402, and 403 to specificpositions so as to thus align them. The movable lens 401 is held by alens holder 410, and the lens holder 410 moves in optical-axisdirections along a linear guide 450 along with the rotation of a ballscrew 412 driven by a motor 411. Likewise, the movable lenses 402 and403 held by lens holders 420 and 430 respectively can move independentlyof each other in the optical-axis directions along the linear guide 450along with the rotations of ball screws 422 and 432 respectively drivenby motors 421 and 431 respectively.

Specifically, movable members 415, 425, and 435 included in alignmentsensors are attached to the distal ends of the lens holders 410, 420,and 430 respectively that hold the movable lenses 401, 402, and 403respectively. Detectors 416, 426, and 436 included in the alignmentsensors are disposed at positions at which the movable lenses 401, 402,and 403 respectively are halted. The motors 411, 421, and 431 are drivenin order to move the lens holders in the optical-axis directions. Thealignment sensors 416, 426, and 436 disposed at positions permitting adesired power detect the movable members 415, 425, and 435 respectivelyso as to align them. Alignment sensors 417 and 418 serve as limitsensors for detecting the upper and lower limits in the optical-axisdirections of a movable range of the movable lens 401. Likewise, limitsensors 427 and 428 and limit sensors 437 and 438 are included for themovable lenses 402 and 403 respectively. As the alignment sensor, anoptical or magnetic sensor is conceivable.

The foregoing movements are performed in response to a command issuedfrom the overall control unit 50. For example, when the circuit patternsformed on the substrate 1 to be inspected are highly dense, a high-powerhigh-sensitivity inspection mode is designated. When the circuitpatterns are less dense, quick inspection is performed with a low power.Thus, the power is appropriately designated so that many microscopicdefects can be detected according to information on the surface of thesubstrate 1 to be inspected which is placed on the stages or accordingto a step included in a manufacturing process.

Next, a description will be made of a conditions-for-inspectiondesignation sequence to be followed by the foreign matter or defectinspection system in accordance with the present invention.

FIG. 12 is a flowchart describing designation of conditions forinspection. The wafer 1 is placed on the table 34 by a loader andimmobilized by performing vacuum absorption (S2800). Chip layoutinformation, that is, information on a chip size of a wafer or presenceor absence of a chip in the wafer is designated (S2801). Thereafter, theentire wafer 1 is rotated so that the directions of arrays of chips 202in the wafer 1 will run parallel to the edges of the photodetectors 26(a rotation error is nearly nullified) (S2802). Thereafter, forinspection of each area on a wafer with optimal sensitivity, aninspective area is designated and detective sensitivity for theinspective area are designated (S2803). Thereafter, optical conditionssuch as a direction of illumination light to be irradiated to the wafer,an angle to be selected, and a power to be selected for thevariable-power detective optical system 20 are designated (S2804). Anoptical filter is designated (S2805). An amount of light to be detectedis designated (S2806). The conditions for signal processing aredesignated (S2807). Inspection is then initiated (S2808).

Next, manipulation to be performed on a detective signal at aninspection step (S2808) will be described below. Signals sent from thephotodetectors 26 having received light reflected and diffracted by thesurface of the wafer 1 are manipulated by the signal processing system40. FIG. 13 shows the configuration of the signal processing system 40.Referring to FIG. 13, the signal processing system 40 includesprocessing circuits 40 a, 40 b, and 95 associated with thephotodetectors 26 a, 26 b, and 92. When the number of detectors isincreased, the number of processing circuits is increased accordingly.

The configuration of the processing circuit 40 a and the actions to beperformed thereby will be described below. The processing circuit 40 aincludes: an A/D converter 1301 that receives a detective signal fromthe detector 26 a; a data memory block 1302 in which a detective imagesignal f(i,j) resulting from the A/D conversion is stored; a thresholdcalculation block 1303 that calculates a threshold on the basis of thedetective image signal; foreign matter detection blocks 1304 a to 1304 neach of which includes as circuits, which detect a foreign matter atevery merger of pixels on the basis of the detective image signal 1410read from the data memory block 1302 and threshold image signals (Th(H),Th(Hm), Th(Lm), Th(L)) provided by the threshold calculation block 1303,pixel merger circuits 1305 a and 1306 a, a foreign matter detectioncircuit 1307 a, and an inspective area handling circuit 1308 a; acharacteristic quantity calculation circuit 1309 that calculates acharacteristic quantity such as an amount of scattered light obtained bydetecting a defect under low-angle illumination, an amount of scatteredlight obtained by detecting a defect under high-angle illumination, orthe number of detected pixels implying a degree of spread of a defect;and an integration block 1310 that classifies defects, which includesmall and large foreign matters on a semiconductor wafer, a defectivepattern, and a micro-scratch, on the basis of the characteristicquantity calculated for each merger by the characteristic quantitycalculation block 1309.

The foreign matter detection blocks 1304 a to 1304 n have the pixelmerger circuits 1305 a to 1305 n and 1306 to 1306 n, the foreign matterdetection circuits 1307 a to 1307 n, and the inspective area handlingcircuits 1308 a to 1308 n respectively associated with respective mergeroperators that handle pixels the number of which corresponds to aproduct of, for example, 1×1, 3×3, 5×5, etc., or n×n.

A signal produced by the photodetector 26 a is digitized by the A/Dconverter 1301. A detective image signal f(i,j) 1410 is stored in thedata memory block 1302 and transferred to the threshold calculationblock 1303. The threshold calculation block 1303 calculates a thresholdimage Th(i,j) 1420 to be used to detect a foreign matter. The foreignmatter detection circuit 1307 detects a foreign matter on the basis ofsignals which the pixel merger circuits 1305 and 1306 have manipulatedaccording to values produced by merger operators of an associated type.

The inspective area handling block 1308 manipulates a detected foreignmatter signal and a threshold image in terms of a place of detection.Concurrently, based on signals produced by the pixel merger circuits1305 a to 1305 n and 1306 a to 1306 n, the foreign matter detectioncircuits 1307 a to 1307 n, and the inspective area handling blocks 1308a to 1308 n included in the respective foreign matter detection blocks1304 a to 1304 n associated with the respective types of mergeroperators, the characteristic quantity calculation circuit 1309calculates a characteristic quantity (for example, an amount of lightscattered under high-angle illumination, an amount of light scatteredunder low-angle illumination, or the number of defect-detected pixels).The integration block 1310 integrates the foreign matter signal with thecharacteristic quantity, and transmits the resultant data to the overallcontrol unit 50.

To be more specific, the A/D converter 1301 is a circuit having theability to convert an analog signal produced by the photodetector 26into a digital signal. The number of bits to be converted preferablyranges from 8 to 12 bits. If the number of bits is small, a resolutionresulting from signal processing is low. This makes it hard to detectfeeble light. On the other hand, if the number of bits is large, anexpensive A/D converter is needed. This leads to the demerit that theprice of the system gets high. The data memory block 1302 is a circuitin which the analog-to-digital converted signal is stored.

Next, the pixel merger circuits 1305 and 1306 will be described inconjunction with FIG. 14.

The pixel merger circuits 1305 a to 1305 n and 1306 a to 1306 n includemutually different merger operators 1504.

The merger operators 1504 are facilities each of which merges detectiveimage signals f(i,j) 1410 that are read from the data memory block 1302and that represent respective pixels arrayed in n rows and n columns,and each of which merges difference threshold image signals 1420, eachof which includes a detective threshold image signal Th(H), a detectivethreshold image signal Th(L), an inspective threshold image signalTh(Hm), and an inspective threshold image signal Th(Lm) that areproduced by the threshold calculation block 1303, concerning therespective pixels arrayed in n rows and n columns. The merger operators1504 are, for example, circuits each of which produces mean valuesrelevant to the pixels arrayed in n rows and n columns.

The pixel merger circuits 1305 a and 1306 a include merger operatorseach of which merges a value relevant to, for example, a pixel arrayedin one row and one column. The pixel merger circuits 1305 b and 1306 binclude merger operators each of which merges values relevant to pixelsarrayed in three rows and three columns. The pixel merger circuits 1305c and 1306 c include merger operators each of which merges valuesrelevant to pixels arrayed in five rows and five columns. The pixelmerger circuits 1305 n and 1306 n include merger operators each of whichmerges values relevant to pixels arrayed in n rows and n columns. Themerger operators that merge a value relevant to a pixel arrayed in onerow and one column provide input signals 1410 and 1420 respectively asthey are.

The threshold image signal includes, as mentioned above, four imagesignals (Th(H), Th(Hm), Th(Lm), and Th(L)). Therefore, each of the pixelmerger circuits 1306 a to 1306 n needs four merger operators Op.Consequently, the pixel merger circuits 1305 a to 1305 n transmit mergeddetective image signals 431 a to 431 n that are the results of merger ofdetective image signals performed by the merger operators 1504. On theother hand, the pixel merger circuits 1306 a to 1306 n transmit mergedthreshold image signals 441 a (441 a 1 to 441 a 4) to 441 n (441 nl to441 n 4) that are the results of merger of four sets of threshold imagesignals (Th(H), Th(Hm), Th(Lm), Th(L)) performed by the merger operatorsOp1 to Opn. The merger operators included in the respective pixel mergercircuits 1306 a to 1306 n are identical to one another.

The advantage of merger of values relevant to pixels will be describedbelow. Foreign matter inspection should detect not only a microscopicforeign matter but also a large thin-film-like foreign matter thatspreads over a range of several micrometers wide. However, a detectiveimage signal acquired from the thin-film-like foreign matter is notalways intense enough. Therefore, a detective image signal representinga pixel exhibits a low signal-to-noise ratio and may therefore be leftunfound. Therefore, every set of pixels arrayed in n rows and n columnsof which size corresponds to the size of the thin-film foreign matter iscut out and convoluted to other set of pixels, whereby thesignal-to-noise ratio is improved.

Next, the inspective area handling blocks 1308 a to 1308 n will bedescribed below.

The inspective area handling blocks 1308 a to 1308 n are employed in acase where data representing an area (including an area within a chip)that need not be inspected is removed from a foreign matter or defectdetective signal acquired from a specific chip by each of the foreignmatter detection circuits 1307 a to 1307 n, in a case where a detectivesensitivity has to be changed area by area (including an area within achip), or in a case where an area to be inspected is selected.

Regarding the inspective area handling blocks 1308 a to 1308 n, forexample, when the detective sensitivity for an area on the substrate 1to be inspected may be low, a threshold for the area calculated by athreshold calculator 1411 included in the threshold calculation block1303 may be set to a large value. Otherwise, data representing a foreignmatter in an area that should be inspected may be extracted from dataitems representing foreign matters, which are produced by each of theforeign matter detection circuits 1307 a to 1307 n, on the basis ofcoordinates representing the position of the foreign matter.

An area for which detective sensitivity may be low is, for example, anarea on the substrate 1 to be inspected where the density of circuitpatterns is low. The merit of lowering the detective sensitivity is thatthe number of foreign matters to be detected is efficiently decreased. Ahigh-sensitivity inspection system may detect as many as several tenthousands of foreign matters. At this time, what is most significant isa foreign matter in an area in which circuit patters exist. Takingmeasures against the significant foreign matter provides a shortcut toimprovement in a yield of device manufacture.

However, when the whole of the substrate 1 to be inspected is inspectedwith the same sensitivity, since a significant foreign matter and aninsignificant foreign matter coexist, the significant foreign mattercannot be readily extracted. Therefore, the inspective area handlingblocks 1308 a to 1308 n are used to lower the detective sensitivity foran area, of which foreign matter do not adversely affect a yield and inwhich no circuit pattern exists, on the basis of CAD information on eachchip or threshold map information. Thus, the significant foreign mattercan be efficiently extracted. However, a method of extracting a foreignmatter is not limited to the method of changing detective sensitivity.Alternatively, foreign matters may be classified as described later inorder to extract a significant foreign matter or the significant foreignmatter may be extracted based on the sizes of foreign matters.

Next, the characteristic quantity calculation circuit 1309 will bedescribed below.

What is referred to as a characteristic quantity is a value representingthe characteristic of a detected foreign matter or defect. Thecharacteristic quantity calculation circuit 1309 is a processing circuitfor calculating the characteristic quantity. The characteristic quantityis, for example, an amount of light reflected and diffracted by aforeign matter or defect and derived from high-angle illumination orlow-angle illumination (an amount of scattered light) (Dh,Dl), thenumber of defect-detected pixels, the shape of a foreign matter-detectedarea, the direction of a principal axis of inertia, a place on a waferwhere a foreign matter is detected, a type of circuit patterns on asubstrate, or a threshold for detection of a foreign matter.

Next, the integration block 1310 will be described below.

The integration block 1310 has the ability to integrate the results offoreign matter detections concurrently performed by the pixel mergercircuits 1305 and 1306 respectively or integrate a characteristicquantity calculated by the characteristic quantity calculation circuit1309 with the results of foreign matter detection (positionalinformation on a foreign matter or defect) and to transmit the result ofthe integration to the overall control unit 50. The integration of theresults of inspection should preferably be performed by a personalcomputer or the like so that the contents of processing can be easilymodified.

The processing circuit 40 b performs the same processing as theforegoing processing circuit 40 a does. An image signal produced by thephotodetector 26 b that has detected light reflected or scattered underhigh-angle illumination is manipulated by the components ranging from anA/D converter 2301 to a results-of-inspection integration block 2310 inorder to detect a foreign matter or defect. The characteristic quantityconcerning the foreign matter or defect is then calculated.

The overall control unit 50 adds up (ORs) foreign matter or defectdetective signals, which have been manipulated by theresults-of-inspection integration blocks 1310 and 2310 so as to specifythe position and characteristic quantity of a foreign matter or defectexisting on the wafer 1. Information on the specified foreign matter ordefect is displayed on the screen of the display means 52 in the form ofa map representing the surface of a wafer or a histogram one of whoseaxes indicate sizes.

On the other hand, an image signal representing bright points in animage that is carried by light reflected and diffracted by repetitivepatterns formed on the wafer 1 and that is picked up by the TV camera 92and viewed at the position of the image plane of a Fourier transform inthe detective optical system 20 is transferred to the signal processingcircuit 95, converted into a digital signal by an A/D converter 3301,and then manipulated as image data by an image data processor 3320. Thesignal manipulated as image data by the image data processor 3320 istransferred to a pattern pitch arithmetic block 3330. A pitch Px betweenadjoining ones of bright points 502 in a reflected and diffracted lightimage is calculated. Data representing the pitch Px of the bright points502 and the image data are transferred to the overall control unit 50,and then transmitted as a signal, with which the pitches Wx and Wybetween adjoining ones of slats included in the interceptive patterns503 and 504 incorporated in the spatial filter 22 are controlled, to aspatial filter control unit 27.

In the present embodiment, while both low-angle illumination based onlaser light and high-angle illumination based on wide-band light areperformed concurrently, the wafer 1 is inspected for foreign matters ordefects. An image carried by light that is a component of lightreflected or scattered from the surface of the wafer 1 as a result ofthe low-angle illumination based on laser light and that is notintercepted by the spatial filter 22 but reflected from the beamsplitter 29 is detected by the photodetector 26 a and transferred to aprocessing circuit. A diffraction pattern produced by light that is acomponent of light reflected or scattered from the surface of the wafer1 after light falls on the wafer at a low angle and that is scatteredfrom repetitive patterns formed on the wafer 1 at intervals of arelatively small pitch is intercepted by the spatial filter and thendetected. Moreover, multi-wavelength light emitted from the lamp lightsource is caused to fall on the surface of the wafer 1 at a relativelyhigh angle. Light reflected or scattered from the surface of the wafer 1is detected separately from light scattered under illumination based onsingle-wavelength laser light. Consequently, during one inspection, bothan area on the wafer 1 having repetitive patterns formed therein atintervals of a relatively small pitch (for example, patterns in a memoryunit) and an area having patterns formed therein at intervals of arelatively large pitch (for example, non-memory patterns in a logic unitor the like) can be inspected in order to detect defects.

Second Embodiment

During foreign matter inspection, even a multilayer wafer having atransparent film (for example, an oxide film) coated over the surfacethereof must be inspected. The multilayer wafer is manufactured byrepeating a step of forming patterns on the transparent film. There isan increasing need for detection of only foreign matters on the surfaceof an oxide film during inspection of a wafer having the oxide filmsformed therein. Fundamentally, by decreasing an illuminating angle α, anadverse effect of pattern-diffracted light or light reflected from asubstrate can be suppressed. However, the decrease in the illuminatingangle α poses a problem in that: light that is a regularly reflectedcomponent of illumination light, that is, forward scattered lightoccupies a majority of light scattered from a foreign matter; an amountof scattered light incident on a detective optical system located abovegets smaller; and the foreign matter cannot therefore be detectedstably.

In the second embodiment, a system shown in FIG. 15 is used to detectlight, which is reflected or scattered from the wafer 1 as a result ofillumination, not only from above the wafer but also from obliquelyabove it. In FIG. 15, an illuminative optical system 10 has basicallythe same elements as the one shown in FIG. 2B and includes a low-angleillumination optical system that includes a laser light source 11 and ahigh-angle illumination optical system that includes a wide-band lightsource 12. Parts assigned the same reference numerals as those shown inFIG. 2B and parts located at the same positions of parts shown in FIG.2B but assigned no reference numeral have the same abilities as thosedescribed in conjunction with FIG. 2B.

In the configuration shown in FIG. 15, laser light emitted from thelaser light source 11, having the diameter thereof enlarged by a beamenlargement optical system 16, and reflected in the direction of a pathL3 by a branching optical element 218 passes through a mirror 251, abeam diameter correction optical system 252, mirrors 253 and 254, acylindrical lens 255, and a mirror 256. The light is then irradiated toan area 201-1 on the surface of a wafer 1 in an illuminating direction250 at an illuminating angle γ in the form of a slit-shaped beam 201. Adetective optical system composed of an optical filter 660, an objectivelens 620, a spatial filter 650, an image formation lens 630, and adetector 640 is disposed in a direction 260 that intersects theilluminating direction 250 and that meets a Y-axis direction at ahorizontal angle ((not shown) and meets the surface of the wafer 1 at adetecting angle θ. The slit-shaped beam 201 is irradiated to the waferin the illuminating direction 250, whereby light laterally scatteredfrom a foreign matter present on the surface of a thin film coated overthe wafer is detected. The light receiving surface of the detector 640and an area on the surface of the wafer to which the slit-shaped beam201 is irradiated have a relationship of image formation. The power ofthe image formation lens 630 is determined so that the light receivingsurface of the detector will cover an entire range illuminated with theslit-shaped beam 201.

On the other hand, light scattered upward from a foreign matter existenton the surface of a thin film coated over a wafer illuminated with theslit-shaped beam 201 in the illuminating direction 250 is converged onan objective lens 21. Moreover, out of light reflected or scattered fromthe wafer 1 due to wide-band illumination light waves emitted fromillumination light emission ends 121 a and 121 b of optical fibers 2102to 2109 shown in FIG. 6 by which light that falls within a widewavelength band and is emitted from an illuminative optical system 12 isbranched into a plurality of paths, upward scattered light is convergedon the objective lens 21. As described in relation to the firstembodiment, the scattered light waves have the wavelengths thereofseparated from each other and are then detected by photodetectors 26 aand 26 b respectively.

A detective system is designed to form an image. This has the meritsthat stray light other than light reflected or scattered from an objectof detection is prevented from giving an adverse effect and thatinspection is speeded up because parallel processing can be performed.An automatic focusing control system that is not shown controls adetector so that the light receiving surface 203 of the detector will bedisposed in a range illuminated with the slit-shaped beam 201. Thus, thesurface of the wafer is, without fail, located at a certain position inthe Z direction during inspection.

As shown in FIG. 15, the present embodiment includes, in addition to adetective optical system whose elements range from the objective lens 21to the photodetectors 26 a and 26 b and which is disposedperpendicularly to the surface of the wafer 1, a first oblique detectionsystem whose elements range from a wavelength selection filter 660 to aphotodetector 640, and a second oblique detection system whose elementsrange from a wavelength selection filter 661 to a photodetector 641. Thedetector 640 included in the first oblique detection system and thedetector 641 included in the second oblique detection system are,similarly to the detectors 26 a and 26 b, realized with TDI imagesensors respectively. Moreover, a spatial filter 650 or 651 having thesame ability as the spatial filter 22 described in conjunction with FIG.1 is disposed along the optical axis of the oblique detection system inorder to intercept light reflected and diffracted by patterns.Furthermore, a thin film that selectively transmits light which has thesame wavelength as laser light emitted from a laser light source 11 iscoated over the surfaces of the wavelength selection filters 660 and 661respectively. Thus, a component of light reflected or scattered from thewafer 1 owing to high-angle illumination achieved by a wide-wavelengthband light source 12 is cut and detected. Lenses 620 and 621 areobjective lenses, and lenses 630 and 631 are image formation lenses.

The position of the second oblique detection system is not limited tothe one shown in FIG. 15 but the second oblique detection system may beopposed to the first oblique detection system. When the second obliquedetection system is opposed to the first oblique detection system, ifthe wafer 1 is illuminated obliquely in the illuminating direction 250,the results of detections performed by the first and second detectionsystems respectively are combined in order to obtain a large number ofpieces of information on reflected or scattered light waves.

As for the illuminating direction, illumination may be performed in adirection 220 or a direction 230. However, an illuminating means and adetective optical system composed of the image formation lens 630 anddetector 640 should preferably be disposed for fear they may beinterfere with each other, and directed or angled in order to avoid anadverse effect of light reflected from the substrate such as lightdiffracted by patterns. Namely, the illuminating means and detectiveoptical system should preferably be located at experimentally determinedoptimal positions.

Image signals detected by the detector 640 included in the first obliquedetection system and the detector 641 included in the second obliquedetection system respectively are manipulated by signal processingsystems 40 c and 40 d respectively. The manipulation of the detectedimage signals is identical to that described in conjunction with FIG.13. An iterative description will be omitted.

Third Embodiment

FIG. 16 shows an embodiment of a defect inspection system including amicroscope. In the present embodiment, a foreign matter detected duringinspection can be verified using an observational optical system 60.

A detected foreign matter (including a quasi foreign matter) on thewafer 1 is moved to a position within a field of view offered by amicroscope included in the observational optical system 60 by movingstages 31 and 32. The foreign matter is then observed.

The advantage provided by the inclusion of the observational opticalsystem 60 lies in a point that a detected foreign matter can beimmediately observed without the necessity of moving a wafer to areviewer such as a scanning electron microscope (SEM). Since a foreignmatter detected by an inspection system can be immediately observed, acause of occurrence of the foreign matter can be identified quickly.Moreover, an image of a detected foreign matter picked up by a TV camera64 included in the observational optical system 60 is displayed on acolor monitor that is shared by a personal computer. Also included is afacility capable of irradiating laser light to part of a wafer centeredon a detected foreign matter, scanning the wafer by moving stages forinspection, marking a scattered light image of the foreign matter andthe position of the foreign matter, and displaying the marked image andposition on a monitor. Consequently, whether a foreign matter isactually detected can be verified. As an image of part of a waferscanned by moving the stages, an image of a die adjoining a die in whicha foreign matter is detected may be picked up. Comparison andverification can be achieved immediately.

As a microscope included in the observational optical system 60, eithera microscope employing a light source that emits visible light (forexample, white light) or a microscope employing a light source thatemits ultraviolet light may be adopted. For observation of an especiallymicroscopic foreign matter, a high-resolution microscope, for example,the microscope based on ultraviolet light is preferred. The adoption ofthe microscope based on visible light provides color information on aforeign matter. This is advantageous in that the foreign matter can bereadily recognized.

Fourth Embodiment

Referring to FIG. 17 to FIG. 19, a description will be made of anotherembodiment that adopts as the low-angle illumination light source 11 anultraviolet laser such as a KrF laser or an ArF laser and that includesthe low-angle illumination optical system 10 included the arrangementshown in FIG. 2B or FIG. 15.

An amount of light scattered from a microscopic particle of 0.1 μm orless in diameter is inversely proportional to the fourth power of thewavelength of illumination light. Therefore, the employment of lighthaving a short wavelength permits higher sensitivity. In order toimprove the sensitivity in detecting a defect, an ultraviolet laseremitting light of a shorter wavelength is adopted as an illuminationlight source. Moreover, for detection of light reflected or scatteredfrom a microscopic foreign matter or defect, an amount of illuminationlight should be large.

When a pulsed oscillation laser is adopted as the ultraviolet laser, apeak value (maximum power) of power provided by the pulsed oscillationlaser is much larger than a required mean power. For example, assumingthat the mean power of a laser is 2 W, the emission frequency of lightemitted therefrom is 100 MHz, the pulse spacing thereof is 10 ns, andthe pulse width thereof is 10 ps, the peak value (maximum power) is solarge as 2 kW that it may damage a specimen. Consequently, the peakvalue (maximum power) should preferably be decreased with the mean powerheld intact.

As for a method of decreasing the peak value with the mean powersustained, as shown in FIG. 17, a laser beam L0 emitted from the lightsource 11 is enlarged by a beam enlargement optical system 16, androuted to a pulse branching optical system 17 so that the laser beamwill be branched into a plurality of paths whose path lengths aredifferent from one another. Thereafter, the light paths are integratedinto one. Thus, one pulse of laser light emitted from the light sourceis divided into a plurality of pulses having a smaller peak value. Theplurality of pulsed laser light waves is routed to a branching opticalelement 218 so that the light waves will be introduced to any of thedirections of paths L1, L2, or L3 shown in FIG. 2B or FIG. 15. Thus, thelight waves are recomposed into a slit-shaped beam which is thenirradiated to a slit-shaped area 201-1 on the wafer 1.

Since a pulsed laser beam is split into a plurality of components andthen irradiated, assuming that the moving speed of an X stage 31-1 onwhich a substrate W to be inspected is placed is 20 cm per sec and thesize of a field of view offered by one pixel location in a photodetector26 a or 26 b is 1 μm, when an ultraviolet pulsed laser beam whosefrequency is 100 MHz is split into a plurality of components under theaforesaid conditions, several hundreds or more of pulses of laser lightare repeatedly irradiated to an area to be detected by one pixellocation in the detector 26 a or 26 b. Consequently, speckle noisesderived from a laser beam can be temporally averaged for imaging. Thisresults in an image having noises minimized.

FIG. 18A shows an example of a pulsed light split optical system 17. Inthis example, the pulsed light split optical system 17 includesquarter-wave plates 1711 a and 1711 b, polarization beam splitters (PBS)1712 a and 1712 b, and mirrors 1713 a and 1713 b. The quarter-wave plate1711 a transforms an incident laser beam, which is enlarged by the beamenlargement optical system 16 and is linearly polarized (p-polarizedlight in this example), into elliptically polarized light. Thepolarization beam splitter 1712 a separates the elliptically polarizedlight into p-polarized light and s-polarized light. One of the polarizedlight waves, that is, the p-polarized light passes through thepolarization beam splitter 1712 a and polarization beam splitter 1712 b.The other s-polarized light is reflected from the polarization beamsplitter 1712 a, mirrors 1713 a and 1713 b, and polarization beamsplitter 1712 b, and returned to the same optical axis as thep-polarized light having passed through the polarization beam splitters1712 a and 1712 b is. At this time, assuming that the spacing betweenthe polarization beam splitter 1712 a and mirror 1713 a or between thepolarization beam splitter 1712 b and mirror 1713 b is L/2 m, the pathof the s-polarized light and the path of the p-polarized light have anoptical path difference of L m. Assuming that the light velocity is cm/s, the s-polarized light and p-polarized light have a temporaldifference expressed by formula 2 below.

t(s)=L(m)/c(m/s)  (2)

A beam containing two pulses that have a time interval T between them asshown in FIG. 18B and being emitted from the laser light source 11 istemporally split. Consequently, as shown in FIG. 18C, one pulse is splitinto two pulses having a time interval t between them in order to halvethe peak value of the pulse.

For example, assume that laser light having a pulse spacing of 10 nm(10⁻⁸ sec) and a pulse width of 10 Ps (10⁻¹¹ sec) is employed and thespacing between the polarization beam splitter 1712 a and mirror 1713 aor between the polarization beam splitter 1712 b and mirror 1713 b isset to 15 cm (0.15 m). In this case, the temporal difference between thes-polarized light and p-polarized light is 1 ns (10⁻⁹ sec).Consequently, a pulsed laser beam whose peak value is halved isirradiated to the surface of a wafer twice at intervals of 1 ns within10 ns.

The angle of rotation of the quarter-wave plate 1711 a is adjusted sothat the ratio of s-polarized light to p-polarized light in a beamincident on the polarization beam splitter 1712 a will be 1:1(circularly polarized light). Due to a loss in reflectance ortransmittance caused by employed optical elements (polarization beamsplitters 1712 a and 1712 b and mirrors 1713 a and 1713 b), the peakvalues of s-polarized pulsed light and p-polarized pulsed lightseparated from a beam emitted from the polarization beam splitter 1712 bbecome different from each other. In order to decrease a maximum valueto be assumed by the peaks of respective pulsed light waves, the peakvalues of the respective pulsed light waves must be equal to each other.

According to the configuration of the pulse split optical system 17shown in FIG. 18A, p-polarized light is affected by the transmittance(Tp) of p-polarized light offered by the polarization beam splitters1712 a and 1712 b. In contrast, s-polarized light is affected by boththe reflectance (Rs) of s-polarized light offered by the polarizationbeam splitters 1712 a and 1712 b and the reflectance (Rm) of s-polarizedlight offered by the mirrors 1713 a and 1713 b. Assuming that Ls denotesa loss of s-polarized light and Lp denotes a loss of p-polarized light,a loss ratio (PI) is expressed by formula 3 below.

PI=Ls/Lp=Rm ² ×Rs ² /Tp ²  (3)

Consequently, when the angle of rotation of the quarter-wave plate 1711a is adjusted so that the ellipticity of polarized light incident on thepolarization beam splitter 1712 a will be equal to the loss ratio, thepeak values of s-polarized pulsed light and p-polarized pulsed lightinto which light emitted from the polarization beam splitter 1712 b isseparated become nearly equal to each other. The p-polarized pulsedlight and s-polarized pulsed light that are separated to have the nearlyequal peak value are transmitted by the quarter-wave plate 1711 b tobecome circularly polarized light waves.

The method of halving pulsed light using the pulse split optical system17 has been described. Referring to FIG. 19A and FIG. 19B, a method ofquartering pulsed light will be described in relation to a variant ofthe pulse split optical system 17 that splits pulsed light into a largernumber of components. The configuration of a pulse split optical system17′ shown in FIG. 19A includes two stages of the pulse split opticalsystem 17 shown in FIG. 18A. The spacing between a polarization beamsplitter 1732 c and a mirror 1733 c or between a polarization beamsplitter 1732 d and a mirror 1733 d included in the second stage is setto a double of the spacing between a polarization beam splitter 1732 aand a mirror 1733 a or between a polarization beam splitter 1732 b and amirror 1733 b included in the first stage. Light emitted from thepolarization beam splitter 1732 b in the first stage includesp-polarized pulsed light and s-polarized pulsed light that lags behindthe p-polarized pulsed light. The optical pulse train is recomposed intocircularly polarized light by a quarter-wave plate 1731 b. P-polarizedlight whose intensity is a half of the intensity of the optical pulsetrain transmitted by the quarter-wave plate 1731 b is transmitted by thepolarization beam splitters 1732 c and 1732 d. S-polarized light whoseintensity is a half of the intensity of the pulse train is reflectedfrom the polarization beam splitter 1732 c and mirrors 1733 c and 1733d, reflected from the polarization beam splitter 1732 d, and returned tothe same optical axis as the p-polarized light is. Consequently, pulsedlight is quartered, and the peak values of the quarters are a quarter ofthe peak value of the pulsed laser beam emitted from the light source11. Strictly speaking, since optical elements causes loses inreflectance or transmittance as mentioned above, the peak value issmaller than the quarter.

In the configuration shown in FIG. 19A, p-polarized pulsed laser lighthaving passed through the polarization beam splitters 1732 c and 1732 dand s-polarized pulsed laser light reflected from the mirror 1733 d andpolarization beam splitter 1732 d propagate along the same optical axis,are recomposed into circularly polarized light by the quarter-wave plate1731 c, and then falls on the polarization beam splitter 1734(equivalent to the branching optical element 218 shown FIG. 2B or FIG.15). Consequently, the circularly polarized light is separated intop-polarized light and s-polarized light. One of the polarized lightwaves, that is, the p-polarized laser light is propagated along a pathL2′ equivalent to the path L2 shown in FIG. 2B or FIG. 15, and thenreshaped by a cylindrical lens 1735 (equivalent to the cylindrical lens244 in the path L2 shown in FIG. 2B or FIG. 15). The resultant lightilluminates a linear area 1793 on the wafer 1 (equivalent to the lineararea 201-1 on the wafer 1 shown in FIG. 2B or FIG. 15).

On the other hand, the s-polarized light reflected from the polarizationbeam splitter 1734 and thus angled 90° is propagated along a path L3′equivalent to the path L3 shown in FIG. 2B or FIG. 15, reflected frommirrors 1736 and 1737 to have the path changed to another, and thenreshaped by a cylindrical lens 1738 (equivalent to the cylindrical lens255 in the path L3 shown in FIG. 2B or FIG. 15). The resultant lightilluminates, unlike p-polarized laser light that illuminates the waferin the direction of the path L2′, the linear area 1793 on the wafer 1perpendicularly to the wafer 1.

The path L2′ and path L3′ are designed so that the optical lengthsthereof will be different from each other. As shown in FIG. 19B,p-polarized laser light and s-polarized light that are irradiated to thelinear area 1793 on the wafer 1 are irradiated at different timings thathave a temporal difference to proportional to an optical pathdifference. Consequently, interference between the p-polarized laserlight and s-polarized laser light that are irradiated to the linear area1793 can be prevented.

Moreover, the photodetector 26 a that detects light reflected orscattered under illumination with the laser light source 11 detectslight reflected or scattered under illumination performed in a directiondeviated by 90° within a period of time during which the photodetectordetects one pixel. Consequently, a variance in detective sensitivityattributable to a difference in an illuminating direction can beminimized. Eventually, a more microscopic foreign matter or defect canbe stably detected. At this time, a direction of detection is thedirection of an arrow 1740. Moreover, as for the photodetector, eitheror both of the detectors 640 and 641 described in conjunction with FIG.15 may be employed.

According to the present embodiment, an ultraviolet pulsed laser beamcan be irradiated to a wafer with the peak value thereof decreased. Aquite microscopic defect having a diameter of about 0.1 μm or less canbe detected without any damage to the wafer.

According to the present invention, an object of inspection isilluminated with single-wavelength light and wide-wavelength band light,which are different from each other in a wavelength band, in efforts tostably detect a defect on the surface of the object of inspection. Aspatial filter having the ability to intercept light diffracted byrepetitive patterns and pass light diffracted by non-repetitive patternsis disposed at a position in a detective optical system at which aFourier transform is observed. A means is included for designatingconditions for the spatial filter on the basis of an image of lightdiffracted by actual patterns which is viewed at the position of aFourier transform, or an image of light diffracted from patterns whichis supposed to be viewed on the image plane of a Fourier transform andis produced based on design data representing patterns formed on thesurface of the object of inspection. Light scattered from the surface ofa specimen and transmitted by the spatial filter is separated into lightcomponents of different wavelengths. An image is detected by a detectorand compared with an image of an adjoining die within the same specimenin order to detect a microscopic defect. Thus, light diffracted bycircuit patterns on a substrate such as LSI patterns is reduced so thata microscopic foreign matter or defect, a foreign matter or defectcausing a short circuit of wiring, or a thin-film-like foreign mattercan be detected quickly and highly precisely.

Moreover, according to the present invention, one pulse of laser lightemitted from a pulsed laser light source designed for low-angleillumination is split into a plurality of pulses in order to decreasethe peak value. The resultant laser light is irradiated to a specimen.Consequently, a high-luminance pulsed laser may be used to detect a moremicroscopic foreign matter. Nevertheless, the more microscopic foreignmatter can be stably detected without any damage to the specimen.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiment is therefore to be considered in all respects as illustrativeand not restrictive, the scope of the invention being indicated by theappended claims rather than by the foregoing description and all changeswhich come within the meaning and range of equivalency of the claims aretherefore intended to be embraced therein.

1. A defect inspection method comprising the steps of: irradiating first illumination light having a first wavelength so as to be unidirectionally elongated at a first tilt angle on an area of a surface of a specimen; irradiating second illumination light, having a plurality of wavelengths, to the surface of the specimen at a second tilt angle larger than the first tilt angle; separating reflected or scattered light from the first illumination light, from light reflected or scattered from the surface of the specimen under both illumination with the first illumination light and illumination with the second illumination light, and picking up a first optical image of the reflected or scattered light from the first illumination light; picking up a second optical image of the light which is reflected or scattered from the surface of the specimen and from which the reflected or scattered light from the first illumination light is separated; manipulating the first optical image and second optical image; and detecting a defect on the specimen using the result of the manipulation performed on the first optical image and the result of the manipulation performed on the second optical image.
 2. A defect inspection system comprising: having a first wavelength; a first illuminator for irradiating light having a first wavelength so as to be unidirectionally elongated at a first tilt angle on a surface of a specimen; a second illuminator for irradiating light, having a plurality of wavelengths to the surface of the specimen at a second tilt angle larger than the first tilt angle; a detective optical system which forms an optical image of light which is reflected or scattered from the surface of the specimen from the light from the first illuminator and the light from the second illuminator; a light separator which separates reflected or scattered light from the surface of the specimen from the first illuminator from reflected or scattered light from the surface of the specimen from the first illuminator and the second illuminator; a first imager for picking up a first optical image of the reflected or scattered light which is separated by the light separator from the first illuminator; a second imager for picking up a second optical image of the reflected or scattered light from which the reflected or scattered light from the first illuminator is separated by the light separator; and a defect detector for manipulating the first optical image and the second optical image so as to detect a defect on the specimen. 